org.erights.e.elang.evm
Class LiteralExpr

java.lang.Object
  |
  +--org.erights.e.elang.evm.ParseNode
        |
        +--org.erights.e.elang.evm.ENode
              |
              +--org.erights.e.elang.evm.EExpr
                    |
                    +--org.erights.e.elang.evm.LiteralExpr

All Implemented Interfaces:

Cloneable, EPrintable, MatchMaker

public class LiteralExpr

extends EExpr

Safe: BNF: LiteralInteger | LiteralFloat64 | LiteralChar | LiteralString
# (perhaps someday): | LiteralTwine

Evaluates to the literal's value, which may be any

• int -- any integer without any size limit
• float64 -- any value in the Java subset of the IEEE double precision standard. The set of values is the same as in the IEEE standard except that there's only one NaN value.
• char -- any unicode character representable in Java. I think this is the same as the 16-bit subset of Unicode.
• String -- a ConstList of any sequence of chars. In E, a String is really a bare Twine.
• (perhaps someday) Twine -- a ConstList of chars + source position information recording where these characters allegedly came from. A bare Twine (ie, a String) is a Twine without any annotations.

Note that not all E scalars are written as literals. In particular, null, false, and true are instead the names of non-shadowable variables in the universal scope holding the corresponding values. These scalars can thereby be written "literally enough" by writing these variable names. The similar cases of Infinity and NaN are explained below.

A LiteralExpr object may be programmatically created to hold any int or float64 value. However, the following ints and float64s cannot be written as literals in the E source language:

• Negative integers. For example, the E expression -3 expands to the Kernel-E 3.negate(). A LiteralExpr holding a -3 prints as -3, which does express the original value in E (by using a call expression).
• Likewise for negative float64s. -3.1 expands to 3.1.negate(). A LiteralExpr on -3.1 prints as -3.1, which does express the original value in E. Likewise for -0.0, which has the same magnitude (<=>) as 0.0 but is not the same (==) as 0.0.
• float64 positive infinity. This is the result of, for example, the expression 1.0/0.0. A LiteralExpr on this value prints as Infinity. This is the name of an unshadowable variable in the universal scope holding this value, so the printed form does express the original value as a variable reference.
• float64 negative infinity, as results from the expression -1.0/0.0. This prints as -Infinity, which expresses the original value.
• The one float64 Nan, as results from the expression 0.0/0.0. This prints as NaN, which is an unshadowable variable in the universal scope. The printed form thereby expresses the original value.

Because these literals have the same meaning as the E programs they print as, an optimizer operating on Kernel-E ASTs may, perhaps as part of constant folding, turn such constant expressions into LiteralExprs holding these values.

Author:

Mark S. Miller

Field Summary

private Object

myValue

Fields inherited from class org.erights.e.elang.evm.ENode

Fields inherited from class org.erights.e.elang.evm.ParseNode

NUM_PR, PR_ASSIGN, PR_CALL, PR_COMP, PR_EEXPR, PR_LISTPATT, PR_ORDER, PR_PATTERN, PR_PRIM, PR_START

Constructor Summary

LiteralExpr(SourceSpan optSpan, Object value)
Enabled: Makes an EExpr that will evaluate to 'value'.

Method Summary

protected StaticScope	computeStaticScope() When staticScope() is first requested on a given node, it calls computeStaticScope() to do the actual computation, which is then remembered.
boolean	matchBind(ConstList args, Object specimen, FlexList bindings) Enabled:
String	printRep() Enabled:
protected Object	subEval(EvalContext ctx, boolean forValue) The recursive part that does the work
void	subPrintOn(TextWriter out, int priority) Enabled:
Object	value() Enabled:
Object	welcome(ETreeVisitor visitor) Enabled:

Methods inherited from class org.erights.e.elang.evm.EExpr

__printOn, appendAllTo, appendTo, eval, eval, evalBool, printAsBlockOn, substitute, transform

Methods inherited from class org.erights.e.elang.evm.ENode

cleanCopy, copy, getOptPatternMap, getOptScopeMap, getPatternMap, getScopeMap, qbuild, setPatternMap, setScopeMap, staticScope, synEnv

Methods inherited from class org.erights.e.elang.evm.ParseNode

asText, getOptSpan, lnPrintOn, lnPrintOn, matchBind, matchBind, matchBind, printListOn, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

myValue

private final Object myValue

Constructor Detail

LiteralExpr

public LiteralExpr(SourceSpan optSpan,
                   Object value)

Enabled: Makes an EExpr that will evaluate to 'value'.

Parameters:

value - If 'value' is a String (though not a Twine), then it is interned during construction. This makes no difference to E's semantics, but 1) may be more efficient, and 2) helps the E language and ELib/Java work together more smootly, since Java also interns all String literals. When compiling E to JVM bytecodes, an E literal will turn into a Java literal anyway, so this anticipates the resulting Java-level semantics.

Currently, 'value' may also be a Character, BigInteger, or Double. We expect to eventually allow 'value' to be a Twine as well, in order to preserve the source position of quasi-strings with Kernel-E code that's still readable.

Method Detail

welcome

public Object welcome(ETreeVisitor visitor)

Enabled:

Specified by:

welcome in class ENode

computeStaticScope

protected StaticScope computeStaticScope()

Description copied from class: ENode

When staticScope() is first requested on a given node, it calls computeStaticScope() to do the actual computation, which is then remembered.

Specified by:

computeStaticScope in class ENode

subEval

protected Object subEval(EvalContext ctx,
                         boolean forValue)

Description copied from class: EExpr

The recursive part that does the work

Specified by:

subEval in class EExpr

matchBind

public boolean matchBind(ConstList args,
                         Object specimen,
                         FlexList bindings)

Enabled:

Specified by:

matchBind in interface MatchMaker

Specified by:

matchBind in class ParseNode

subPrintOn

public void subPrintOn(TextWriter out,
                       int priority)
                throws IOException

Enabled:

Specified by:

subPrintOn in class ParseNode

IOException

printRep

public String printRep()

Enabled:

value

public Object value()

Enabled: